This invention relates to coatings systems, such as for cookware, which minimize sticking. More particularly, it relates to multilayer coating systems that can be used on smooth, untreated substrates.
Generally in the art a metal or glass substrate is roughened by some means before the first layer of coating is applied so that mechanical bonding will assist chemical adhesive means in holding the coating onto the substrate. Typical roughening means include acid etching, sand-blasting, grit-blasting, and baking a rough layer of glass, ceramic or enamel frit onto the substrate. The problem of adhesion of non-stick coatings to substrates is exacerbated by the nature of the coatings. If the coating is optimized for release to prevent food particles from sticking to it, easy clean-up after cooking, durability or to facilitate low friction sliding contact, almost by definition there will be difficulties in making it adhere well to the substrate.
The substrate can be metal, often aluminum or stainless steel used for cookware or industrial applications. It can be glass or ceramic. It might even be plastic for microwave oven cookware, or it could be an industrial article such as a saw made of carbon steel. Whatever the substrate or the application, if it is necessary to roughen the substrate to make the coating adhere, that at least adds cost and can cause other difficulties including creating a rough profile which can protrude or telegraph through the coating. This is especially undesirable when smoothness is sought, such as for saws and steam irons. The environmental cost of disposing of etchant materials can be significant. Sometimes, especially for glass and ceramic substrates, it also can cause unacceptable weakness or brittleness of the substrate.
Means of enhancing adhesion of non-stick coatings to a substrate are illustrated by the following patents.
U.S. Pat. No. 2,562,118-Osdal (1951) uses a mixture of phosphoric and chromic acids in a coating containing polytetrafluoroethylene (PTFE) for use directly on metal or glass. This can cause some inherent etching of the substrate. Better adhesion would be desirable.
U.S. Pat. No. 4,049,863 - Vassiliou (1977) teaches a primer containing fluoropolymer, such as PTFE, colloidal silica and a polyamide imide (PAI), along with other constituents, applied by various techniques to a substrate that is preferably pretreated by grit blasting, flame spraying of metals or metal oxides or frit coating, or to phosphated and chromated metals. The PTFE:PAI ratio can be 1:9. The primer coat is ordinarily applied to a dry film thickness (DFT) of about 2-15 microns (.mu.m). After air drying, the primer is topcoated with a conventional fluoropolymer enamel and baked. (Parts, percentages and proportions herein are by weight except where indicated otherwise.)
Although U.S. Pat. No. 4,259,375 - Vassiliou (1981) teaches applying a decorative fluoropolymer coating on a primer which is applied directly to the substrate, the implication is that the usual roughened substrate is used.
U.S. Pat. No. 3,801,379 - Blackwell (1974) applies a poly(arylene sulfide) coating such as polyphenylene sulfide (PPS) directly to an aluminum substrate after treating the substrate with hot water or steam. Preferably the coating is done with the substrate at 455.degree. C., causing flocking of the coating. The thickness of the coating is not disclosed, and the PPS can contain 5-20% PTFE.
Canadian 887,122 - Fang (1971) provides a single coating of PAI and PTFE on a metal substrate with a concentration gradient from mostly PAI at the substrate to mostly PTFE at the air interface. This is applied as a single coat without any special primer on ordinary steel or sand-blasted aluminum.
British 1,512,495 (SEB) (1978) applies a PAI coating containing PTFE powder at a DFT of 5-100 microns to a metal or glass substrate that has been merely de-greased and not roughened. The minimum ratio of PTFE:PAI is 1:7. This then is the undercoat on which a PTFE powder topcoat is applied. However, the PTFE in the undercoat at these thicknesses can lead to failure of intercoat adhesion.
European application 100,889 - Gebauer (1984) describes thicker coatings containing certain perfluorocarbon resins, PPS, lithium hydroxide, and silica and asserts they are superior to prior art primer coatings on the order of 10 microns thickness. If the substrate is smooth, he teaches that it can be roughened by sand blasting or etching.
U.S. Pat. No. 4,177,320 and U.S. Pat. No. 4,139,576 - both Yoshimura, et al, (1979) teach an undercoat of PPS or PPS plus PAI or polyimide with surface active agents on sand-blasted rough aluminum at a DFT of 15 or 20-25 microns. As in many of these patents, the need for roughening the substrate to obtain adequate adhesion and durability adds considerable costs.
Coatings systems including primers, intermediate and top coats useful with the present invention are described in various patents including:
U.S. Pat. No. 4,049,863 - Vassiliou (1977);
U.S. Pat. No. 4,118,537 - Vary and Vassiliou (1978);
U.S. Pat. No. 4,123,401 - Berghmans and Vary (1978);
U.S. Pat. No. 4,252,859 - Concannon and Vary (1981);
U.S. Pat. No. 4,351,882 -Concannon (1982); all incorporated herein by reference.